1. Field of the Invention
The present invention relates to a technique for connecting a plurality of optical function devices formed on the same substrate, in particular, to a connecting method capable of suppressing a leakage of stray light from an optical input side to an optical output side, and an optical apparatus applied with the connecting method.
2. Related Art
There have been demanded developments of optical communication systems and optical signal processing systems capable of constructing networks of large capacities and ultra-long distance with an explosive increase of Internet Protocol (IP) data communication demand. In a transmission system adopting a wavelength-division multiplexing (WDM) transmission as a basic technique thereof, it is possible to realize the large capacity transmission and easily perform the division-multiplication with the wavelength as a unit, so that the construction of flexible optical networks that performs multiplication-division of different kinds of services at optical levels, such as, optical cross-connecting (OXC), optical add/drop multiplexing (OADM) and the like. Therefore, the development and manufacturing of transmission apparatus and signal processing apparatus using the above system have been remarkably made.
In these apparatuses, there are utilized many optical function devices, such as, an optical switch having functions for switching ON/OFF of light, for attenuating the light, for switching to 1×n, or the like, a wavelength filter that separates a signal light for each wavelength, or the like. Specifically, the optical switch (including an attenuator) is used, for example, for adjusting the levels of respective wavelengths at a wavelength division multiplexing section on the sending side, for Automatic Level Control (ALC) controlling by an optical amplifier, for wavelength switching in the OXC and OADM, for switching ON/OFF of light and the like. Further, the optical filter is used, for example, for wavelength switching in the OXC and OADM, for separating the respective wavelengths on the receiving side, for cutting off Amplified Spontaneous Emission (ASE) light and the like.
By forming these optical function devices on a substrate made of SiO2, LiNbO3 and the like, it becomes possible to achieve the high functions, down-sizing, integration, reduction of electricity, and reduction of cost. The plurality of optical function devices integrated on the substrate are used individually in parallel with one another, for example, as shown in FIG. 17, or are cascade connected in a multi-staged structure to be used, for example, as shown in FIG. 18A and FIG. 18B, so as to achieve the respective functions thereof. Thus, in a case where the respective optical function devices are used individually in parallel with one another, an effect owing to the integration becomes large. Further, in a case where the respective optical function devices are cascade connected in a multi-staged structure to be used, it becomes possible to achieve the improvement of extinction ratio, if the optical function devices are optical switches, while if the optical function devices are optical filters, such as, acousto-optic tunable filters (AOTFs), it becomes possible to achieve the narrow transmission band, the improvement of suppression ratio between other channels, and the improvement of extinction ratio when used as notch filters. Moreover, if devices having different functions from one another are cascade connected in a multi-staged structure to be used, it becomes possible to achieve a high function and the like.
However, in a case where a plurality of optical function devices integrated on the substrate are connected to be used, most of the input light from a substrate input section passes through the optical function devices, however, as shown by an arrow in dotted line in FIG. 17 and FIGS. 18A and 18B, a part of the input light is emitted into the substrate, and bypasses the optical function devices as a stray light, to be coupled to an output section. The coupling of this stray light to the output section causes deterioration of extinction ratio, in a case where the optical function devices are optical switches. Further, in a case where the optical function devices are optical filters, such as AOTFs and the like, the coupling of this stray light to the output section causes deterioration of suppression ratio between other channels or deterioration of extinction ratio at the time when the AOTFs are used as notch filters.
Specifically, as one example, the consideration is made on an optical switch having characteristics in which an optical insertion loss is 10 dB and an extinction ratio at an optical function portion is 40 dB, as shown in a solid line in FIG. 19, in a case where a level difference of stray light to an input light is 40 dB, the extinction ratio of the optical switch is about 30 dB. In a case where the level difference of stray light to an input light is 50 dB, the extinction ratio is about 37 dB. Thus, the extinction ratio of the optical switch is restricted by an influence of stray light. If a required value of extinction ratio of the optical switch is assumed to be 40 dB, the required value cannot be achieved even if the level difference of stray light to the input light is 50 dB or more. Therefore, it is necessary to make the stray light level negligibly smaller compared to the extinction ratio at the optical function portion.